How is "Activate this only once per turn" different to ? Okay, technically you can do this once on your turn and once on the opponent's, but that's not really enough to prevent this being back to being worse than Mana Cylix again.

That's what I get for using Obelisk of Bant as a straw man, eh? I should point out, though, that Chromatic Lantern isn't a good example either, since it's a rare, and we're talking about the common slot with this card. Manalith is probably what we both should have been pointing to.

"More powerful than Obelisk of Bant" doesn't really work, because so are loads of modern cards: Chromatic Lantern is the most recent. But yes, a mana stone costing is right out. Most mana stones should cost and ETBT, or cost and ETB untapped.

The Skyshroud Elf version of mana filtering is also arguably a bit good (and in fact this is stronger even than that, filtering to 3 colours not just 2). With two of these you can run Elemental Appeal, Dawn Elemental and, um, Invoke Prejudice and expect to cast any of them on turn 4. That's probably a bit too strong these days.

To put it another way, Wizards thinks that Charcoal Diamond, and the rest of the two casting cost artifacts that produce one mana, are too powerful to print nowadays. This card, as printed, is more powerful than Obelisk of Bant (albeit in different colors).

On a separate note, I've always found it interesting that, while Mana Cylix is a very underpowered card (I'd probably last pick it in a Conflux draft, with a few exceptions, and never put it in my deck, with a few exceptions) it still has a rating of 3.250 on Gatherer. A lot of people like the card, even if it isn't very good. I don't think you have to push hard to make a lot of people like a three color mana-filterer. This is a step too far. Maybe something a little closer to Temple of the False God? ": Add to your mana pool. You may use this ability only if you control five lands."?